The Avg Lead Time field represents the average number of days from when the item is ordered to when it is received.
Each Purchase Order date is compared to the date when the Order is converted to a Purchase Invoice. The result is divided by the Total Received for the Inventory item to calculate the value for this field.
If Purchase Orders are not used (i.e., purchases are entered directly as Purchase Invoices), the value in this field is zero.
Inventory Database - Order Data Screen Fields